In Long Term Evolution (LTE) Release 13 a mechanism was introduced for the User Equipment (UE) to be suspended by the network. This suspended state is similar to RRC_IDLE. However, unlike the RRC_IDLE state, the UE stores the Access Stratum (AS) context or Radio Resource Control (RRC) context. This makes it possible to reduce the signaling when the UE is becoming active again by resuming the RRC connection, instead of establishing the RRC connection from scratch, as in prior releases. Reducing the signaling may have several benefits such as reducing latency (e.g. for smart phones accessing the Internet) and/or reducing signaling leads such that battery consumption is reduced for machine type devices sending very little data.
The Release 13 solution is based on the UE sending an RRCConnectionResumeRequest message to the network and receiving an RRCConnectionResume message from the network in response. The RRCConnectionResume message is not encrypted but integrity protected.
As part of the standardized work on Fifth Generation (5G) New Radio (NR) in the Third Generation Partnership Project (3GPP), it has been decided that NR should support an RRC_INACTIVE state with similar properties as the suspended state in LTE Release 13. The RRC_INACTIVE has slightly different properties from the suspended state in that it is a separate RRC state and not part of RRC_IDLE as in LTE. Additionally the Core Network (CN)/Radio Access Network (RAN) connection (Next Generation (NG) or N2 interface) is kept for RRC_INACTIVE, whereas it was suspended in LTE.
FIG. 1 illustrates possible UE state transitions in NR. The properties of the states illustrated in FIG. 1 are as follows:
RRC_IDLE:                A UE specific Discontinuous Reception (DRX) may be configured by upper layers;        UE controlled mobility based on network configuration;        The UE:                    Monitors a Paging channel for CN paging using 5G System Architecture Evolution (SAE) Temporary Mobile Subscriber Identity (TMSI) (5G-S-TMSI);            Performs neighboring cell measurements and cell (re-)selection;            Acquires system information.                        
RRC_INACTIVE:                A UE specific DRX may be configured by upper layers or by RRC layer;        UE controlled mobility based on network configuration;        The UE stores the AS context;        The UE:                    Monitors a Paging channel for CN paging using 5G-S-TMSI and RAN paging using Inactive Radio Network Temporary Identifier (I-RNTI);            Performs neighboring cell measurements and cell (re-)selection;            Performs RAN-based notification area updates periodically and when moving outside the RAN-based notification area;            Acquires system information.                        
RRC_CONNECTED:                The UE stores the AS context;        Transfer of unicast data to/from UE;        At lower layers, the UE may be configured with a UE specific DRX;        For UEs supporting Carrier Aggregation (CA), use of one or more Secondary Cells (SCells), aggregated with the Special Cell (SpCell), for increased bandwidth;        For UEs supporting Dual Connectivity (DC), use of one Secondary Cell Group (SCG), aggregated with the Master Cell Group (MCG), for increased bandwidth;        Network controlled mobility, i.e. handover within NR and to/from Evolved Universal Mobile Telecommunications Service (UMTS) Terrestrial RAN (E-UTRAN);        The UE:                    Monitors a Paging channel;            Monitors control channels associated with the shared data channel to determine if data is scheduled for it;            Provides channel quality and feedback information;            Performs neighboring cell measurements and measurement reporting;            Acquires system information.                        
In LTE, an RRC_CONNECTED UE can be suspended by receiving an RRCConnectionRelease message with a suspend indicator. Upon receiving that message, the UE stores some parameters and deletes others. Some of these stored parameters, provided by the source node that is suspending the UE, are used by the UE when the UE attempts to resume the connection.
More particularly, upon receiving an indication to be suspended, the UE stores the AS context including the RRC configuration used in RRC_CONNECTED and the following parameters associated with the last source Primary Cell (PCell) the UE was connected to when the UE was in RRC_CONNECTED:                Cell Radio Network Temporary Identifier (C-RNTI) of the last PCell in RRC_CONNECTED;        Cell Identity (28 bits value that identifies a cell within a Public Land Mobile Network (PLMN)) of the last PCell in RRC_CONNECTED;        Physical Cell Identity (PCI) of the last PCell in RRC_CONNECTED.        
Further details relating to how the UE behaves according to the LTE standard in response to receiving an RRCConnectionRelease message are found in Appendix A.
When the RRC_IDLE UE with suspend configuration wants to resume (i.e., in LTE), the stored parameters (C-RNTI, Cell Identity, and PCI) associated with the last PCell the UE was connected to when the UE was in RRC_CONNECTED are used in the RRC Resume procedure to compute the short Message Authentication Code for Integrity (MAC-I) security token so that the UE can be recognized by the source node hosting the UE AS context. This enables the source node to accept a context fetch request from the target node. Further details relating this procedure may be found in Appendix B.
There currently exist certain challenge(s). In NR RRC, which is different from LTE RRC, the network may respond to a ResumeRequest from the UE with a Suspend message (or equivalent, such as a Release message with a suspend indication or configuration) which immediately orders the UE back to RRC_INACTIVE state. LTE does not permit sending a suspend message (e.g., a release message with a suspend indication) directly to the UE trying to resume the connection, as in the example shown in FIG. 2. Rather, this feature is new in NR.
In NR RRC, the network may alternatively respond to a ResumeRequest from the UE with a Release message (i.e., without a suspend indication) which immediately orders the UE back to RRC_IDLE state. This message is encrypted. LTE does not permit sending a release message directly to the UE trying to resume the connection, as in the example shown in FIG. 3. Rather, this feature is also new in NR.
Currently known procedures for RRC connection handling (e.g., in NR draft specifications) adopt features similar to LTE. However, RRC connection handling when a UE is re-suspended are not well developed or understood.